Allow filtering the columns that are returned

[waldoj]

In #17, @wdoerner proposes the ability to filter output either positively ("include these columns") or negatively ("exclude these columns"), writing:

There are times when you might have many columns and keep=name would avoid displaying all of them. Likewise, drop=address could tighten up the display of a dataset where address was not desired (if there are many fields, it's just quicker to do a drop than to type in all the fields for a keep). A flexible function would be one that reads in multiple elements (maybe enclosing with square brackets or quotes) if a user wants to keep or drop more than a single field, like keep=[address,city,state]. Another approach would be a joint command like keep=address&keep=city but that performs them both at the same time instead of executing either keep=address or keep=city (I've found that two sort commands would execute only one or the other but not both so I make the distinction here for clarity).

(I think we could skip the brackets, and just use a comma-separated list, which I've had luck with in the past.)